1. Field of the Invention
The present invention relates to the preparation of ultrapure boehmites and pseudo-boehmites from amorphous aluminum hydroxycarbonate, to the boehmites thus prepared and to the various uses or applications thereof.
2. Description of the Prior Art
Aluminas can be in a variety of crystalline forms, depending upon the particular process employed for their preparation. The industrial applications of alumina demand, in certain cases, that it should have the maximum active surface area and the smallest possible crystallite size.
Aluminas having a high specific surface area frequently consist of alumina monohydrates. Same are typically obtained from aluminum salts (nitrate, chlorides), such as described, for example, in French Pat. No. 1,261,182. The products obtained are generally used as hardeners, coating agents, starting materials for catalysts, catalyst supports and desiccants.
One of the principal disadvantages of the aforesaid process is the need to use, as the source of alumina, materials which are difficult and uneconomic to prepare and which give rise to ecologically hazardous effluents, such as aluminum chloride or nitrate solutions obtained from aluminum metal.
Furthermore, the corrosive nature of the anions used frequently results in contamination of the product alumina by release of metal values forming part of the apparatus of preparation.
The anion which is most easily displaced from alumina and which presents the least toxicity hazard is the carbonate ion, which is thermally unstable. It was therefore envisaged to prepare aluminum hydroxycarbonate by carbonating alkali metal aluminates with carbon dioxide or alkali metal bicarbonate. The carbonate is then displaced in order to provide the alumina.
Thus, it is known from U.S. Pat. No. 3,268,295 to produce an alumina hydrate having the crystal structure of pseudo-boehmite and containing from 1.4 to 1.6 mols of water per molecule of alumina. This product has a greater specific surface area than that of activated alumina. It is prepared by carbonating sodium aluminate to give a precipitate of an amorphous alumina gel, washing this precipitate and then thermally decomposing same at a temperature above 100.degree. C. to provide a crystalline alumina hydrate. In contrast to fibrillar boehmite, the product obtained is not dispersible in water.
It too is known, from European patent application No. 15,196, assigned to the assignee hereof, to prepare alumina partially in the form of boehmite. As the starting material, this process uses an active alumina of poorly crystalline and/or amorphous structure, obtained by the rapid dehydration of aluminum hydroxides. It proves relatively difficult to obtain very pure products by this process.
Cf. U.S. Pat. Nos. 2,898,307 and 3,839,536; French Pat. Nos. 969,488 and 2,449,650; British Patent Specification No. 1,143,787; Chemical Abstracts, 84, No. 6, page 3, item 35867z (Feb. 9, 1976) and Chemical Abstracts, 83, No. 8, page 276, item 62890n (Aug. 25, 1979).